The present invention relates to a filter apparatus for removing impurities from a gas and for adjusting the humidity of a gas. The present invention further relates to an exposure apparatus used in the photolithography step of a manufacturing process for various devices such as a semiconductor element, a liquid crystal display element, an imaging element, a thin-film magnetic head and the like. The present invention further relates to a device manufacturing method for manufacturing various devices.
In this type of exposure apparatus, an illumination optical system illuminates a mask, such as a reticle or a photomask, having a predetermined pattern with predetermined exposure light. Further, the exposure apparatus includes a projection optical system for projecting an image of the predetermined pattern onto a substrate (e.g., wafer, glass plate), to which a photosensitive material such as photoresist is applied, when illuminated by the illumination optical system. The illumination optical system and the projection optical system, which includes a plurality of optical elements such as a lens element and a mirror, are accommodated in a barrel.
The exposure apparatus is a very delicate apparatus and the temperature in the apparatus must be kept constant for each part of the exposure apparatus to function properly. In the prior art, the exposure apparatus is installed in a clean room of which the room temperature is adjustable, and the temperature distribution in the exposure apparatus is uniform due to the air, of which the temperature is controlled, drawn into the exposure apparatus from the clean room.
In such an exposure apparatus, progress has been made to shorten the wavelength of the exposure light so as satisfy the strong demand for miniaturization of circuit patterns in recent years. For instance, an exposure apparatus using a KrF excimer laser (λ=248 nm) in the far ultraviolet range or an ArF excimer laser (λ=193 nm) in the vacuum ultraviolet range as the exposure light has been recently developed.
However, the following problems arise when using exposure light having a short wavelength. Gas of an organic substance reacts with oxygen, water vapor, carbon hydride gas or exposure light in the space (e.g., the internal space of the barrel) through which the exposure light passes and produces a clouding substance on the surface of an optical element, such as a lens element, and acts as a light absorption substance that absorbs the exposure light.
In particular, when used as the exposure light, light having a short wavelength that is less than or equal to the wavelength of the ArF excimer laser, the absorption of the exposure light by the light absorption substance is greater than the exposure light of ultraviolet light, such as an i-line. Therefore, the energy of the exposure light may be significantly lowered before the exposure light reaches the substrate from the light source. In this manner, the throughput of the exposure apparatus is lowered and the product yield is lowered when the energy of the exposure light itself decreases or the transmissivity of the exposure light decreases due to the clouding of the optical element.
In the prior art, a chemical filter capable of removing the light absorption substance is arranged in the exposure apparatus to prevent the throughput from being lowered. The chemical filter removes the light absorption substance in the gas drawn into the space, including the light path for the exposure light. In order to perform temperature control in the exposure apparatus with higher accuracy, an exposure apparatus that has been developed draws air from the clean room into the exposure apparatus in a state in which the fluctuation range relative to the target temperature is controlled to be smaller. However, it has become apparent that even if the temperature of the air supplied to the chemical filter is adjusted in advance so as to be kept substantially constant, the fluctuation range relative to the target temperature of the air drawn into the exposure apparatus is enlarged compared to before the air passes through the chemical filter.
This is considered to be because of the exchange of moisture that occurs between the air and the chemical filter when the air passes through the chemical filter. More specifically, the chemical filter has a property for containing moisture at an amount in which the humidity of the chemical filter is balanced with the humidity of the gas. That is, the chemical filter has a property for containing more water as the humidity of the gas increases. Therefore, if air is heated by a temperature controller to decrease the relative humidity when adjusting the temperature of the air, the chemical filter disperses moisture and evaporative latent heat is removed from the chemical filter. As a result, the temperature of the air after passing through the chemical filter becomes lower than the temperature of the air before entering the chemical filter.
On the other hand, if the air is cooled with a cooler to increase the relative humidity when adjusting the temperature of the air, the chemical filter adsorbs and takes in the moisture thereby generating adsorption heat. As a result, the temperature of the air after passing through the chemical filter becomes higher than the temperature of the air before entering the chemical filter. Thus, the temperature of the air changes by passing through the chemical filter even if the temperature of the air is adjusted to a predetermined temperature before passing through the chemical filter.
The temperature of a clean room is presently adjusted with satisfactory accuracy. However, in many cases, the humidity of the clean room is not sufficiently adjusted in terms of the control range or control cycle etc. Further, an enormous equipment investment would become necessary to perform humidity management in a large clean room with satisfactory accuracy.
A technique for connecting an environment controller to an exposure apparatus and controlling the environment inside the exposure apparatus has also been proposed (refer to for example, Japanese Laid-Open Patent Publication No. 2002-158170). In the environment control apparatus, a plurality of chemical filters for removing a light absorption substance are arranged along the circulation direction of the air. In the exposure apparatus, the amount of moisture exchanged between the air and the chemical filter decreases towards the downstream in the circulation direction of the air. Therefore, the fluctuation change relative to the target temperature of the air passing through the environment control apparatus is small, and the temperature of the air is substantially maintained at a target value.
There are many types of chemical filters including those made of activated carbon, ion exchange fabric etc., and the amount of moisture exchanged between air and the chemical filter differs depending on the material of the chemical filter. Therefore, in an exposure apparatus connected to the environment controller, the type of chemical filter must be selected comprehensively taking into consideration various aspects such as the capacity for removing the light absorption substance, the amount of moisture exchanged between the air and the chemical filter and the like. This may narrow the chemical filters that can be selected.
Further improvement in the exposure accuracy of the exposure apparatus is necessary for exposure apparatuss to correspondence with exposure light that will have even shorter wavelengths in the future. In order to satisfy this requirement, the temperature of the air drawn into the exposure apparatus must be more stable, and it is essential to control not only the temperature but also the humidity of the air with higher accuracy.